Components typically of automobiles are manufactured by subjecting steel sheets to press working. Burring (hole expanding) and/or bending is carried out in the press working process. The material steel sheets subjected to press working desirably have hole-expandability (also called “stretch-flangeability”) and/or bendability. The material steel sheets subjected to press working also desirably have shape freezing ability, so as to fabricate components having desired shapes with good dimensional accuracy. In particular, pillars and other structural components of automobiles should have further increased strength so as to improve collision safety of automobiles, and high strength thin steel sheets having tensile strengths of 980 MPa or more are to be adopted thereto.
Accordingly, material steel sheets subjected to press working should have both high tensile strengths of 980 MPa or more and satisfactory press workabilities such as hole-expandability, bendability, and shape freezing ability.
Japanese Unexamined Patent Application Publication (JP-A) No. 2006-161111 discloses a technique of providing a steel sheet having an increased strength and improved hole-expandability (stretch-flangeability). This steel sheet has a ferrite single phase microstructure and contains elements which form carbide precipitates, such as Ti, Nb, V, and Mo, in the microstructure to cause precipitation strengthening to thereby have an increased strength. However, as is described in this document, the steel sheet having an increased strength due to precipitation strengthening has an increased yield ratio and thereby has deteriorated shape freezing ability.
JP-A No. 2005-248240 proposes a technique of providing a hot rolled steel sheet having a strength exceeding 490 MPa and showing improved hole-expandability (stretch-flangeability). This technique achieves a strength on the order of exceeding 490 MPa and gives improved hole-expandability (stretch-flangeability) by allowing the steel sheet to have a microstructure mainly containing bainitic ferrite and by suitably controlling the average grain size of prior-austenite grains. This document also teaches that hole-expandability (stretch-flangeability) can be improved by suppressing localization of elements that cause intergranular embrittlement, such as phosphorus (P), in a system in which precipitation of coarse carbides is suppressed by the action typically of titanium (Ti).
Although giving consideration to hole-expandability (stretch-flangeability), the document (JP-A No. 2005-248240) does not give consideration to bendability and shape freezing ability of steel sheets.